The invention relates to chromium oxide polymerization catalysts and to methods of preparing and using them.
It is known that supported chromium oxide polymerization catalysts can be used to prepare olefin polymers in a hydrocarbon solution to give a polymer having excellent physical and chemical properties. Such catalysts can also be used to prepare olefin polymers in a slurry process in which polymer is produced in the form of small particles of solid material suspended in a diluent. In such polymerization processes, control of the melt index of the polymer can be effected by varying polymerization temperature; higher reaction temperatures generally increase melt index. However, this expedient is limited in particle-form polymerization to about 110.degree. C. in a diluent such as isobutane, as a temperature higher than this causes polymer dissolution, thus negating the purpose of the slurry process, and results in fouling of the reactor due to polymer deposition.
Another way of increasing the melt index of polymers prepared in a slurry process is by increasing the activation temperature of the catalyst. The higher the activation temperature of the catalyst, the higher the polymer melt index. However, the improvement noted is relatively small and increased catalyst activation temperature can narrow the molecular weight distribution of the polymer produced and lower its environmental stress crack resistance. Also, the method is limited by the sintering temperature of the silica-containing support, e.g. about 980.degree.-1095.degree. C. Thus, most polymers produced in a catalytic process represent a compromise between the melt index potential of the catalyst and catalyst activity, both increased by high catalyst activation temperature, and environmental stress crack resistance, which is favored at low catalyst activation temperatures. High activation temperatures have the additional disadvantage of increasing the cost of catalyst preparation. It is therefore desirable to find polymerization catalysts which have a high melt index capacity but do not require high activation temperatures.